We examined the spatial and seasonal variation of Q10 as an indicator of the temperature sensitivity of soil respiration based on field measurements at a young ponderosa pine plantation in the Sierra Nevada Mountains in California. We measured soil CO2 efflux and soil temperature and moisture in two 20m¿20m plots from June 1998 to August 1999. The Q10 values calculated from soil temperature at 10-cm depth ranged spatially from 1.21 to 2.63 among 18 chamber locations in the plots. Seasonally, the Q10 values calculated on the basis of the average soil CO2 efflux and temperature (10 cm) across the sites could vary from 1.05 to 2.3. Q10 and soil temperature are negatively correlated through a simple linear relationship with R2 values of 0.45, 0.40, and 0.54 for soil temperature at 5-, 10-, and 20-cm depth, respectively. However, Q10 and soil moisture are positively correlated with R2 values of 0.81, 0.86, and 0.51 for soil temperature at 5-, 10-, and 20-cm depth, respectively. Q10 values derived from temperatures at different soil depths also showed considerable variation along the vertical dimension. Q10 had a large seasonal variation with the annual minimum occurring in midsummer and the annual maximum occurring in winter. Seasonal values of Q10 depended closely on both soil temperature and moisture. Soil temperature and moisture explained 93% of the seasonal variation in Q10. The spatial variation of Q10 had significant influences on the estimation of soil CO2 efflux of the ecosystem. These variations tended to affect the seasonality of the soil CO2 efflux more than the annual average. The variations of Q10 and its dependence on soil moisture and temperature have important implications for regional and global ecosystem carbon modeling, in particular for predicting the responses of terrestrial ecosystems to future global warming. ¿ 2001 American Geophysical Union